1. Field of the Invention
The present invention relates, in general, to an optical amplification apparatus and, more particularly, to an optical amplification apparatus which employs a plurality of optical amplification stages.
2. Description of the Prior Art
In general, a stimulated brillouin scattering-phase conjugate mirror (SBS-PCM) reflects a phase conjugated wave to compensate for the distortion of a laser beam which occurs in a laser amplification procedure. Further, since the SBS-PCM ensures easy arrangement of an optical system, the SBS-PCM is used for a light splitting laser having a high output.
An optical amplification system employing such an SBS-PCM is disclosed in Korean Patent No. 318520.
Hereafter, a conventional optical amplification system will be described with reference to FIG. 1a. 
FIG. 1a illustrates a conventional optical amplification apparatus.
Referring to FIG. 1a, a light 505, which is output from a laser oscillator 500, is amplified into a gradually increasing number of laser lights while passing through a first optical amplification stage 510, a second optical amplification stage 540, and a third optical amplification stage 570. In detail, the light 505 is reflected by a polarizing beam splitter (PBS), is incident upon a first optical isolator 520, is reflected by an SBS-PCM, is transmitted through the PBS, and is incident upon a first optical amplifier 530. Then, the light which is incident upon the first optical amplifier 530 is amplified while being transmitted through the amplifier, is reflected by an SBS-PCM, and is incident upon the PBS. The light which is incident upon the PBS is reflected and output toward a beam expander (BE) 535. The beam expander 535 enlarges the size of the incident light and outputs the incident light to the second optical amplification stage 540. The second optical amplification stage 540 comprises a second optical isolator 550 which is constructed in the same manner as the first optical isolator 520 of the first optical amplification stage 510 to perform the same function. The second optical amplification stage 540 additionally includes a wavefront division type beam splitter 562 positioned at the front end of a second optical amplifier 560 which forms 2×2 arrays. Here, the wavefront division type beam splitter 562 is used to deliver lights to the respective 2×2 arrays of the second optical amplifier 560. That is to say, before the light is incident upon the second optical amplifier 560, the light is wavefront-divided by the wavefront division type beam splitter 562. These lights are amplified by respective amplifiers arranged along optical axes, are reflected by SBS-PCMs, are re-coupled with each other, and are incident upon a PBS. The PBS reflects and outputs the incident light to a beam expander 565. The beam expander 565 enlarges the size of the incident light and outputs the enlarged incident light to the third optical amplification stage 570. In the third optical amplification stage 570, a third optical amplifier 590 forms 4×4 arrays, and a third optical isolator 580 forms 2×2 arrays. Here, wavefront division type beam splitters 582 and 592 are used to deliver lights to the respective arrays.
Due to the fact that the optical amplification system is constructed in this way, by adding one or more optical amplification stages as desired, output energy can be increased and repeatability can be maintained as it is without causing damage to an optical system. In this regard, when the energy density of a laser light increases due to continuous amplification, if the energy density is not prevented from increasing, damage to the optical system and a laser gain medium is caused. Thus, when it is necessary to enlarge the size of the laser light, the size of the laser gain medium must simultaneously be enlarged. This results in retardation of the cooling velocity of the laser gain medium, and actually makes it impossible to create a laser light having high repeatability. In this consideration, the optical amplification apparatus capable of re-coupling lights shown in FIG. 1a, which can increase the size of the laser light and maintain the size of the gain medium, is used.
The beam splitter employed in the optical amplification apparatus capable of re-coupling the lights as shown in FIG. 1a comprises a wavefront division type beam splitter which is shown in FIG. 1b. 
Referring to FIG. 1b, the wavefront division type beam splitter divides an incident light ‘a’ into two output lights ‘b’. Namely, in the wavefront division type beam splitter, a laser light is divided into two small-sized laser lights.
In addition to the wavefront division type beam splitter, an amplitude division type beam splitter has also been disclosed in the art, which is shown in FIG. 1c. 
Referring to FIG. 1c, the amplitude division type beam splitter divides an incident light ‘a’ into two output lights ‘b’ in a manner such that one output light is transmitted through the beam splitter and the other output light is reflected by the beam splitter. That is to say, in the amplitude division type beam splitter, the size of a laser light is not changed and only energy is shared by the two output lights. In comparison with this amplitude division type, in the wavefront division type which is adopted by the conventional optical amplification apparatus, since the shape of a laser light cannot be the same as that of a light from a main resonator, difficulties are caused in that a cross-section of a gain medium of an amplifier must be formed in the shape of a laser light. The reason for this is that, when a cross-sectional shape of the gain medium is different from the shape of the laser light, amplification efficiency decreases. Further, since a spatial frequency is included in the spatial distribution of a laser light, when reflecting the laser light using an SBS-PCM, breakage of a phase conjugate phenomenon results in. Also, when re-coupling lights after amplification, in the case that phases of two lights at a position where one light meets the other light are different from each other, an intensity spike occurs, which deteriorates the spatial distribution of the lights.